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The question of plant
The principal question asked by everyone with
ambitions to grow hardy and semi hardy tropical plants in the UK is “How hardy
is it?”. Most available information is either US based, or far too sketchy, or
even contradictive. ( For USA Zone Hardiness information please
For USA Zone Hardiness information please click here)However it is a complex question as to how much frost a plant will take and in my own 40 years of experience I am still regularly surprised by the damage or non damage to slightly tender plants after a winter has passed.
The other consideration is to grow jungle plants for
tropical effect but treat them as tender perennials, either just for a single season, or over
wintered in a frost free Conservatory, greenhouse or even indoors. Many of these grow
very quickly from seed, given some early heat in February, making a spectacular
show during the summer in a mixed border or as a specimen plant. It is important
to remember that most tropical plants find our summers too cold to grow properly
outdoors but there are species that fall in the semi hardy category that do well
during our summer and I have tried to include these in my Web Site.
Going back to the question of winter hardiness and if for
example the Chamerops Humilis (Mediterranean fan palm) will tolerate at the
outside minus 10C what does that really mean for our recent winters? I have tried
to simplify the question in the following tabular for m.
Factors effecting temperature
First of all the country location is clearly a factor :-
Then there are specific local climatic influences :-
The specific orientation of the planting :-
On top of these is the question of
a really localised “micro climate” and these can be influenced by
following Table shows a rough rule of thumb as to how these combined factors
might influence localised night winter temperatures around a plant.
If the minus 10C in this case is compared to a front porch in London, we gain 2C for being in Southern England, 2C for a South facing position, 5C for being in a large Urban Mass and a further 5C for being against a sheltered House Wall with an overhang. The likely temperature on the same night on the London front porch was probably no worse than +4C i.e. a 14C hike in temperature! Which explains why geraniums often survive in London and flower outside all Winter but in my own garden some Hebes were killed by the frost and a number of “hardy trees” badly frost damaged!
The story is much
more complex than that of course and to be accurate the health of the plant in
question, the soil type, how dry or wet the conditions were, how long the
temperature stayed below zero etc would need to be considered but the table
above should give you a rough guide to have a chance of success. For example the
Chamerops Humilis already mentioned would most likely prove to be completely
hardy in London, or the West Country or elsewhere given the shelter of a south
facing wall. In fact it has proven hardy so far albeit marginally in my
Oxfordshire front Garden in a sheltered position but with minimal microclimate
Use the Table below
to factor the local temperatures
(assuming my minus 10C typical worse case) for your planting area to
determine the viability of any plant, knowing its temperature hardiness frost
tolerance. Just put the appropriate Degree variance for each row in the right
hand column and compare the total to a typical worse case minus 15C for Central
England. This will give a guide to determine your specific microclimate worst
case winter temperature. I would like to state that this is
only a guide, completely subjective and based on my experiences
with no evaluated scientific basis.
It is still up to you to experiment!